This invention relates generally to a system for differentiating material characteristics using an imaging system. More particularly, it relates to a detector module for use with a hybrid-scintillator/photo sensor and direct conversion imaging system.
In at least one known computed tomography (CT) imaging system configuration having single and/or multi slice scintillator/photodiode rays, an x-ray source projects a fan-shaped, or cone-shaped, beam which is collimated to lie within an x-y-z volume of a Cartesian coordinate system. That x-y-z volume is generally referred to as an “imaging volume” and usually includes a set of x-y planes generally referred to as the “imaging planes.” An array of radiation detectors, wherein each radiation detector includes at least one detector element, is disposed within the CT system so as to receive this beam. An object, such as a patient, is disposed within the imaging plane such that the x-ray beam passes through the object. As the x-ray beam passes through the object being imaged, the x-ray beam becomes attenuated before impinging upon the array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is responsive to the attenuation of the x-ray beam by the object. In turn, each detector element produces a separate electrical signal responsive to the beam attenuation at the detector element location. These electrical signals are referred to as x-ray “attenuation measurements”.
In addition, the x-ray source and the detector array may be rotated by means of a gantry situated within the imaging volume and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, or, “projection data”, from the detector ray at one gantry angle is referred to as a “view.” A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and the detector array. In an axial scan, the projection data is processed so as to construct an image that corresponds to two-dimensional slices taken through the object.
One method of reconstructing an image from a set of projection data is referred to as the “filtered back-projection technique.” This process converts the attenuation measurements from a scan into discrete integers, ranging from −1,024 to +3,072, called “CT numbers” or “Hounsfield units” (HU). These HUs are used to control the brightness of a corresponding pixel on a cathode ray tube or a computer screen display in a manner responsive to the attenuation measurements. For example, an attenuation measurement for air may convert into an integer value of −1,000 HUs (corresponding to a dark pixel) and an attenuation measurement for very dense bone matter may convert into an integer value of +3,000 (corresponding to a bright pixel), whereas an attenuation measurement for water may convert into an integer value of 0 HUs (corresponding to a grey pixel). This integer conversion or “scoring” allows a physician or a technician to determine the density of matter based upon the intensity of the computer display and thus locate anatomical landmarks and identify areas of concern.
Typically, radiation detector rays that are used in imaging systems, such as the T imaging described herein, include single and/or multi-slice scintillator/photodiode etectors. A scintillation detector is constructed of scintillation material, such as cadmium tungstate (CDW04) or rare earth ceramics and operates by receiving x-ray photons emitted by an x-ray source and by converting these x-ray photons into a digital signal that is proportional to the attenuated x-ray energy received. These digital signals are then processed and turned into image data.
One goal of CT imaging is to utilize multi-energy scanning techniques to differentiate tissues and/or materials having varying atomic number and densities, such as calcium and/or iodine. Historically, this has been accomplished using an imaging system having a scintillation detector either by taking single slice images with a single slice CT imaging system having two different x-ray beam filters, or by taking single slice images with a single slice CT imaging system having two different x-ray tube kVp's that exactly overlap spatially, but at a slightly later time, and then processing these two images to separate materials having varying atomic numbers and densities, using suitable known methods such as image subtraction.
For example, using a single slice CT imaging system, a first single slice image would be obtained. The x-ray kVp or the filter at the x-ray tube would then be changed and a second single slice image would be obtained at the same patient location. As mentioned above, the two slices of image data would then be processed to separate the materials of varying atomic numbers and densities within the obtained sliced plane.
Unfortunately, this is an expensive, time consuming and involved process and although a CT imaging system having a direct conversion (DC) detector could conceivably be utilized as the CT imaging system for performing the above-mentioned process, the DC detector would not be able to count the x-rays fast enough to support CT flux rates and/or scan times. Thus, if used in the current mode, the obtained information would suffer from a very high amount of non-linearities that would be very difficult or even impossible to correct in order to achieve artifact free scanning.
The above discussed and other features and advantages of the embodiments will be appreciated and understood by those skilled in the art from the following detailed description and drawings.